Furnaces with bipolar electrodes for the production of metals, particularly aluminum, through electrolysis of molten salts, equipped with auxiliary heating facilities

ABSTRACT

A furnace with bipolar electrodes for the production of metals, particularly aluminum, through the electrolysis of molten salt baths. Electrical heating resistors, selected from graphite fabrics, tapes and felts, are placed in the &#39;&#39;&#39;&#39;cold&#39;&#39;&#39;&#39; zones of said furnace.

United States Patent De Garab [451 May 30, 1972 FURNACES WITH BIPOLAR ELECTRODES FOR THE PRODUCTION References Cited UNITED STATES PATENTS ELECTR0LSIS 0F MOLTEN SALTS 2,773,825 12/1956 Newcombe 4 ..204/274 X 9 3,271,277 9/l966 Yntema ..204/243 X EQUIPPED WITH AUXILIARY 3,382,166 5/1968 De Varda ..204 244 HEATING FACILITIES 3,514,520 5/1970 Bacchiega et al. ..204/243 R X [72] Inventor: Giorgio Olah De Garab, 18, Via Angera, p i E i j h HI Mack Milan, Assistant ExaminerD. R. Valentine [22} Filed: 22, 1969 Attorney-Curt M. A very, Arthur E. Wilfond, Herbert L.

Lerner and Daniel .I. Trek [21] Appl. No.: 859,790

[57] ABSTRACT [30] Foreign Application Priority Data A furnace with bipolar electrodes for the production of metals, particularly aluminum, through the electrolysis of mo]- Sept. 24, ten Salt baths. Electrical heating resistors selected from g phite fabrics, tapes and felts, are placed in the cold" zones of [52] US. Cl. ..204/243 R, 204/274 said furnace [51] Int. Cl. .C22d 3/02, BOlk 3/00 [58] Field of Search ..204/243-247, 274 7 Claims, 2 Drawing Figures e 7 5 5 7 j A 5 7 9 7%? I C l 1 /I 4: j Z 1' 1 1 Q v2.6 3 KIL4\ 2 i A t 4// -l/ T f I FURNACES WITH BIPOLAR ELECTRODES FOR THE PRODUCTION OF METALS, PARTICULARLY ALUMINUM, THROUGH ELECTROLYSIS F MOLTEN SALTS, EQUIPPED WITH AUXILIARY HEATING FACILITIES The present invention concerns equipment for heating the bottom and/or the walls of furnaces for the production of metals, and particularly of aluminum, through electrolysis in molten salt baths. It is known that conventional furnaces for producing aluminum through the so-called igneous electrolysis require no external heat. Consequently there is no problemfor their heating.

The recently developed multicell furnace types for electrolysis in molten salts, provided with bipolar electrodes, can however present such a problem as in these furnaces the molten aluminum produced as well as its neighboring zones of the bath have no longer available, to prevent the cooling efiect, the Joule heat due to the current flowing through a cathode bottom upon which the aluminum collects, as in the traditional furnaces. The lack of a cathode bottom, in multicell furnaces for electrolysis of molten salts by bipolar electrodes, results in some zones being remote from the electrode system of said furnaces. These zones are to be considered as cold zones or, more properly, zones too cold to keep the aluminum gathered in the collecting pit or pits in sufi'iciently flowable condition and to keep the molten salt layers neighboring said molten aluminum in liquid and fluid state without any exceptional thermal insulation or supply of external heat. Thus by cold zones are meant zones which have a temperature equal to or lower than the temperature at which a bath, having a given composition, starts solidifying, that is at a temperature at which the bath tends to thicken and/or solidify. For cryolite baths which are commonly used for the electrolytic aluminum production and which have specific gravity lower or higher than the molten aluminum, according to whether they are in liquid, solid, semi-solid state, the zones at a temperature between 800 and 900 C or lower, can be considered cold as above meant. The presence of bath layers, thickened or solidified, even if partially, at the collecting zones of the produced aluminum, can make the tapping of theproduced metal rather difiicult, if not impossible. Solidification of the bath in said zones can also cause the rising of the molten aluminum. Thus, continuous aluminum layers, in this case, can form on the furnace bottom, correspondingly with two or several bipolar electrodes, in consequence of which a current by-pass or shunt occurs through the same aluminum layers.

in bipolar electrode furnaces for the electrolysis of molten salts, one had the problem of collecting the aluminum produced into pits possibly situated in hot zones of the furnace. It is not always easy to provide and utilize such zones for the intended purpose. To this end,heating systems based on live metal resistors situated in suitable recesses of the pertinent zones of the bottom or walls of the furnace have been tried. These metal resistors present, however, some drawbacks owing both to their mechanical stiffness and easy corrosion. The mechanical stiffness makes them unsuitable for facing the severe strains which the construction materials of the furnace have to undergo at the starting stage and on run. The strains are caused by thermal expansion, impregnation by bath and electrochemical parasitic reactions. The easy corrosion and destruction of the resistors sooner or later occur in consequence of the bath seeping into the furnace structures through cracks and disjointednesses occurring during the run. Moreover, any recess decreases the compactness of the furnace inner structures, consequently increasing the leakage possibility of the molten materials. All the above specified.

drawbacks can quickly damage said metal resistors.

The present invention obviates the above specified inconveniences by supplying with additional heat the bottom and/or the walls of the furnace of the type as above and intended to produce aluminum or other metals by means of resistors chemically inert towards the electrolytic baths, having practically neglectable overall dimensions and practically unlimited flexibility, which allows their adjustment to the sometimes heavy deformation sufiered by the materials of the walls and the bottom of the furnace pot. As electrical resistors to embody in the furnace structures, corresponding to the zones where the additional heat is required, according to the present invention, particularly fabrics, tapes or felts made of graphite fibers, sandwiched between refractory material layers, are foreseen.

Graphite fabrics, tapes and felts are known commercial products and consist of woven or felted filaments made of graphite with 99.9 percent or higher purity, obtained through heat treatment of suitably selected and prepared organic fibers at temperatures reaching about 2,700 C. By the graphite fabrics, tapes or felts according to the present invention, an important technical problem and remarkable technical prejudices have been now overcome.

The characteristics of the present invention can be better deduced from the following description that offers a preferential but not exclusive embodiment, illustrated by the enclosed drawing, as non-limitative example, wherein:

FIG. 1 shows a longitudinal section of a furnace of the type described in de Varda et al. patent application Ser. No. 809,852, filed Mar. 24, 1969, provided with the heating equipment according to the present invention; and

FIG. 2 shows a cross section taken along line A-A in FIG. 1.

With reference to the drawing, one sees a refractory material pot l with double flight stepped down bottom having in the center a pit 2 where liquid aluminum 4, produced by the electrolysis of molten salts 6 in cells 7 formed by bipolar electrodes 5 and terminal electrodes 8, 9 and falling from overflow edges 3, collects. The bottom of this furnace has a special refractory lining 10. This special refractory can be, for example, silicon nitride bonded silicon carbide. The bottom of pit 2 has two thicker layers 11 and 12 made of refractory material, with a thin tape of fibrous graphite 13, approximately as wide as pit 2, interposed between the thicker layers. The direct or alternating current feeding is shown at 14 in FIG. 2. Current flows through studs 15 into two carbon blocks 16 which can be a part of pit 1 or external to it and which are in electrical contact with graphite tape 13, leading therein the electrical current for the resistance generated heating. The carbon blocks 16 are of course electrically insulated from the furnace body of insulating layers 17. g

A temperature ranging from 920 to 930 C and sufficient to keep both the aluminum as well as the neighboring molten salts in molten and. fluid state is thus obtained within pit 2 wherein aluminum collects. This is true notwithstanding the fact that the pit 2 is in the less thermally insulated zone of the furnace, which zone is the thinnest and nearest to the external shell of the furnace. This is important to prevent the danger of bath thickening or freezing (i.e. solidification), which consequently causes clogs in the tapping system for the produced aluminum. The tapping system has been omitted because of the schematic nature of the drawing.

As already mentioned above, the heating resistors to be used in conformity with this invention consist of fibrous graphite in the form of fabrics, tapes or felts of commercial type. The resistors can for example be high purity graphite fabric, having a percentage of ashes of about 0.04 percent, woven with 1,440 filaments y'arn, each filament having a diameter of 8 microns, the weight of the fabric being'2-3 g/m The electrical features are functions of known parameters, i.e. the characteristics of the graphite fabric and the characteristics of the furnace as well as the electrolysis bath. The power taken may indicatively, but not limitatively, amount to about 6 kW/30 dm, i.e. 0.20 kW/dm, (3 kW/30 dm i.e. 0.10 kW/dm as minimum and 9 kW/30 dm i.e. 0.30 kW/dm as maximum values), the electric resistivity of the graphite fabric being equal to E =0.0042 9 cm (at 20 C); the maximum voltage can reach 40-50 V, with preferable use of alternating current at industrial frequency.

The installation consists in laying the graphite fabric, tape or felt on the refractory layer or bricks duringthe erection of the refractory pot, thereon superimposing then the other refractory layer and supplying the suitable electric connections. In the case of a furnace provided with two 30 cm wide pits it is for example suitable to insert two tapes 13 (one for every pit) between refractory layers 11 and '12, connecting them electrically in series, each tape consisting of three graphite fabric layers superimposed one upon another. This is only an example of absolutely non-limiting nature. Between the fibrous graphite resistor and the refractory material laid upon it and/or between 'two or several layers of refractory bricks situated between the aluminum in the pit and the resistor, one or several layers of some millimeters thickness, of silicon carbide of fine grit quality, can be advantageously inserted in order to prevent molten aluminum coming from the pit from seeping downwards and consequently making contact with the disclosed resistor. The use of silicon carbide grit to prevent the escaping of aluminum is described in Italian Pat. No. 790,333. According further to the present invention, it is preferable to smear an electrically conducting paste on the graphite fabric particularly when adopting the embodiment shown in FIG. 2 of the drawing, with carbon blocks 16 for carrying the heating current. The paste shall in this case be smeared between the graphite fabric and the carbon blocks, to improve the electrical contact. Graphite, pitch and tar can be its chemical base; an example of suitable compound, always exemplifying but non-limiting, is:

22 by weight 26 by weight 23 by weight 29% by weight Total:

I claim:

1. in a furnace with bipolar electrodes for the production of aluminum through the electrolysis of molten salt baths, and provided with pits for collecting the molten metals, the improvement which comprises electrical heating resistors, selected from graphite fabrics, tapes and felts, placed at said pits to heat said pits.

2. The furnace of claim 1, wherein the electric resistors are placed in the fumace structure between at least two refractory material layers.

3. The furnace of claim 2, wherein the resistors, interposed between refractory material layers, are placed under the bottom of said collecting pits and have flat surfaces.

4. The furnace of claim 3, wherein the lead-ins of the current to said resistors consist of carbon blocks placed within the walls of the bath containing pot, being the surface of said blocks, facing the pot inside, lined with electrically insulating refractory materials.

5. The furnace of claim 4, wherein an electrically conducting paste is interposed between said graphite resistors and said carbon blocks.

6. The furnace of claim 5, wherein the electrically conducting paste is basically composed by graphite, pitch and tar.

7. The furnace of claim 1, wherein a layer of silicon carbide grit is interposed between said electrical resistors, placed under the bottom of said collecting pits, and the superimposed refractory material layer. 

2. The furnace of claim 1, wherein the electric resistors are placed in the furnace structure between at least two refractory material layers.
 3. The furnace of claim 2, wherein the resistors, inTerposed between refractory material layers, are placed under the bottom of said collecting pits and have flat surfaces.
 4. The furnace of claim 3, wherein the lead-ins of the current to said resistors consist of carbon blocks placed within the walls of the bath containing pot, being the surface of said blocks, facing the pot inside, lined with electrically insulating refractory materials.
 5. The furnace of claim 4, wherein an electrically conducting paste is interposed between said graphite resistors and said carbon blocks.
 6. The furnace of claim 5, wherein the electrically conducting paste is basically composed by graphite, pitch and tar.
 7. The furnace of claim 1, wherein a layer of silicon carbide grit is interposed between said electrical resistors, placed under the bottom of said collecting pits, and the superimposed refractory material layer. 